A. Field of the Invention
This invention relates to improvements in valving systems and, in particular, to systems for preventing leakage of fluids past butterfly step valves.
B. Prior Art
Valving systems employing butterfly valves or step-valves of the butterfly type are used widely throughout industry for many different applications. One application is for thermal regeneration apparatus such as that shown in U.S. Pat. No. 3,895,918 issued to James H. Mueller on July 22, 1975. In that system, a number of heat-exchange sections are arranged about and in communication with a central, high-temperature combustion chamber. Each heat-exchange section includes a heat-exchange bed with a large number of refractory elements or "stones" confined within a heat-exchange bed by inward and outward perforated retaining walls. An industrial effluent to be purified is applied to an inlet duct ring which has branch ducts that distribute the effluent to selected ones of the heat-exchange sections whenever its associated inlet valve is open. In such a case, the effluent traverses the heat-exchange bed which has a very hot front or inner wall that abuts the extremely high temperature produced within the central combustion chamber. The opposite perforated wall of the heat-exchange bed is much cooler being more remote from the central chamber and there is a gradient from high to low temperature between the two walls.
All of the heat-exchange sections are also coupled by branch conduits to an exhaust duct ring, the ring itself being connected to an exhaust fan that draws the gaseous contents of the exhaust ring out and applies them to an exhaust stack or equivalent.
Initially, the effluent traverses a first heat exchange bed in one of the heat exchange sections after passing through an open inlet valve (the outlet valve of that same section being kept closed) and then is drawn through the central combustion chamber where it is purified by high temperature oxidation. It is then sucked through at least a second heat exchange bed to whose stones the purified combustion products lose their very high heat. In the second heat exchange section, during the same interval, the inlet valve remains closed whereas its outlet valve leading to the exchaust ring is open.
When the next cycle begins, however, the second heat exchange section, which has been heated during the previous cycle by the exiting effluent, may have its role reversed (by appropriate control of its valves) so as to function as an inlet heat-exchanger. Conversely, the first heat-exchange section may have its role reversed to function as an outlet heat-exchanger. Thus, in the next cycle, the first heat-exchange section will have its outlet valve turned on and its inlet valve closed, whereas the second heat-exchange section will have just the opposite valve condition. Before the next cycle begins, however, there is an intermediate hiatus interval in which both valves of the first section (inlet and outlet) will be turned off to permit any residual effluent in that section to be drawn off through the combustion chamber. This is to prevent the possibility that this residual unpurified effluent will be drawn directly into the outlet exhaust ring without traversing the heat-exchange bed in the first section when the valves in that first section are reversed in condition during the second cycle. This residual effluent would also have escaped traversal of the central combustion chamber and the heat-exchange bed in the second heat exchange section. Consequently, there would be a risk of emission of noxious or dangerous gases into the atmosphere via exhaust.
The valves used at the inlet and outlet of the respective heat-exchange sections are often metal-to-metal, mainly because of the high temperatures involved. For various reasons, including possible excessive heat at times, the seal afforded by these valves when in the nominally "closed" condition, may be less than perfect. As a result, it is possible, in the hiatus between consecutive cycles of operation, that effluent from the industrial process may leak past a closed outlet valve or closed inlet valve, when that valve is nominally closed. The effluent might go directly into the exhaust duct and out through the stack to the ambient atmosphere or to whatever point in the system that supposedly purified and cooled exhaust gases may be recycled.
While such leakage in many cases may not be very significant, occasions occur in which the effluent has highly toxic or corrosive components. Even the slightest amount of leakage of these components into the ambient atmosphere, or to an exhaust-recycle point, may pose dangers to operating personnel, to the public outside of the plant and cause anti-pollution authorities to take action.
Another effect of such leakage may be to damage the valves downstream because of their corrosive or other chemically active components or may damage the exhaust fan itself since those harmful elements have not been removed by the combustion chamber.
Still another effect of this partial leakage is the reduction of the overall thermal efficiency of the system.
Measurement of leakage of valves is an arduous task. Once a valve has been installed, there is no very practical way to measure the leakage before the valve is put to actual operating conditions. In advance of installation, the testing of leakage of an individual valve on a test stand using ambient air is not very valid because ambient air is at a small fraction of the operating gas temperatures in actuality. Simulation of actual operating temperatures would require elaborate heat-exchange equipment and other expensive equipment. Furthermore, even if a practical test could be devised, each one would have to be individually tested since shop machining practices and allowable tolerances may be unsatisfactory. Two valves supposedly having the same leakage rate may, in fact, have sufficiently different rates that controlled leakages of 1% or less cannot be guaranteed in specifications or attained in comply with anti-pollution laws.
Two ways are known of combating this leakage. One of them involves the use of two valves in series at the inlet and outlet to each heat-exchange bed. This reduces the pressure differential across each valve and thereby the rate and volume of leakage. This is described and claimed in U.S. Pat. No. 4,252,070 to Edward H. Benedick. While this method may be useful in reducing leakage, it does require the use of a double number of valves and appurtenant controls.
A second approach is set forth in U.S. Pat. No. 4,248,841 also to Edward H. Benedick in which relatively pure gas, such as the purified effluent, is fed back to blanket the upstream side of the valves thereby tending to minimize the chances that unpurified effluent can pass the valve.
It is therefore among the objects of the present invention to:
1. Provide a system for minimizing or preventing leakage of unpurified effluent across valves in incineration systems of the type described.
2. Provide an anti-leak system for incineration apparatus which is less expensive than other known systems.
3. Provide a system for preventing the flow of a fluid (gas) past the valve of a valve assembly when said valve is in the nominally closed position.
4. Provide a system for maintaining the thermal efficiency of the apparatus while preventing leakage of unpurified effluent to exhaust.
5. Provide a heat-regenerative incineration system that does not require the use of dual valving and appurtenant controls to maintain extremely low levels of unpurified effluent sent to exhaust.